feat: portfolio tracker — track owned items with buy price, quantity, and P&L

## Problem

Users have no way to track what they own, at what price they bought, and whether they are in profit or loss across their collection.

## Requested behaviour

- Add items to a portfolio with: buy price, quantity, and optional notes
- Dashboard shows: current price, current value, P&L per item and total
- Separate from the watchlist — portfolio = owned items, watchlist = items to monitor

## Acceptance criteria

- [ ] Add to portfolio from item view (price + quantity input)
- [ ] Portfolio sidebar or section with all owned items
- [ ] P&L calculated as (current price − buy price) × quantity
- [ ] Total portfolio value displayed
- [ ] Data persists in `browser.storage.local`